I. Field of the Invention:
This invention relates to novel thermoplastic copolyesters and adhesive compositions formed therefrom.
II. Brief Description of Prior Art:
In the case of solution adhesives, it is necessary that the adhesives be applicable to a substrate from a solution, and, in the case of hot melt adhesives, that the adhesives have high thermal or heat stability at elevated temperatures (i.e. the ability to resist degradation or gelation at high temperatures over time). Both types of adhesives require good wetting characteristics for good adhesion and low viscosity for ease of application. Hot melt and solution adhesives in general should also be characterized by high heat resistance or heat distortion temperature (i.e. the ability to form an adhesive bond which withstands high temperatures without softening or losing adhesion and adherence), and cold resistance (i.e. the ability to form a flexible adhesive bond which withstands low temperatures without becoming brittle), good cohesive strength, color (preferably clear or light), and compatibility with a large variety of tackifying resins, plasticizers, solvents, modifiers, pigments and the like.
In formulating such adhesives, it has been found that good low temperature properties are usually associated with polymers having a low glass transition temperature, high heat resistance with crystalline or semi-crystalline polymers, and good cohesive strength with both high molecular weight polymers and crystalline or semi-crystalline polymers.
In selecting copolyesters for use in adhesive formulations it is further desired that such copolyesters be easily and economically prepared. In much of the conventional copolyester manufacturing operations, high temperatures (up to 300.degree. C.), high vacuum (0.1-1.0 mm Hg) and long polycondensation times are required to obtain products exhibiting useful adhesive characteristics. Such extreme conditions do not always result in the best obtainable products, due to degradation reactions occurring during the polycondensation, and in addition increase the cost of manufacture.
Copolyesters of ethylene glycol, terephthalic acid and dimer acid as well as similar copolyesters have been known and used in specific adhesive formulations such as those described, for example, in U.S. Pat. Nos. 3,383,343; 3,390,108; 3,329,740; 3,975,323 and French Pat. No. 1,398,551.
Copolyesters have also been prepared from ethylene glycol, terephthalic acid and particular substituted succinic acids as described in U.S. Pat. No. 3,542,737. These copolyesters are prepared using polycondensation temperatures of 275.degree. C., at which temperature succinic compositions tend to decompose (i.e. decarboxylate), thus causing chain termination and so lowering the molecular weight of the final copolyester. Moreover, the patent teaches the use of such percentages of the succinic constituent (i.e., 5-15 mole percent based on the moles of acid units in the copolyester) that, in actuality, the succinic constituent plays very little part in the overall reaction. Furthermore, the copolyesters prepared in accordance with the teachings of the patent are useful only as fibers, filaments or films and, due to the relatively short chain length between functional groups in the succinic components, have such a high rigidity in the backbone that they would not be thought useful in the production of adhesives requiring, for example, low glass transition temperatures.
It is an object of the present invention to provide a novel thermoplastic copolyester useful, alone or in combination with typical adhesive formulations, as an adhesive composition.
It is a further object to provide an adhesive exhibiting good thermal stability, high heat and cold resistance, good wetting characteristics, low viscosity, good cohesive strength and a high compatibility with a large variety of tackifying resins, plasticizers, solvents, modifiers, pigments, etc.
Another object is to provide a thermoplastic copolyester which may further be cured through functional groups (e.g., hydroxyl or carboxyl groups) to improve their strength, heat and solvent resistance.
It is also an object to provide a method for manufacturing such thermoplastic copolyesters wherein mild conditions involving temperatures no higher than 265.degree. C., pressures 1-30 mm Hg and relatively short polycondensation times of 2-7 hours are employed.